This contrast doesn’t just matter for Trump’s candidacy. It reveals a startling political splintering of conservative Catholics and evangelicals, who have tracked side by side for decades.

This alliance of Catholics and evangelicals came about in the 1970s. Before that, Catholics were a solid Democratic Party voting bloc, comprised largely of the immigrant underclass. Economic and social status, not moral issues, determined Catholic voting.

As the Democratic Party pushed social issues — brought to prominence in George McGovern’s presidential campaign in 1972 — and then the Roe v. Wade Supreme Court decision in 1973 stunned many antiabortion supporters into political action, Republican strategists saw an opportunity to transform U.S. politics by aligning evangelicals and antiabortion Catholics.

This alliance was not easy to achieve given past divisions between evangelicals and Catholics, most prominently in the presidential candidacy of John F. Kennedy in 1960. Kennedy had to pledge to Protestant ministers that if he were elected, he would exercise independent judgment and not take direction from the Vatican. In the 1960s, Southern Baptists often led opposition to state proposals to restrict abortion rights, because Catholic-led organizations, such as the National Right to Life Committee, were promoting such legislation. Evangelicals for years opposed public funding for private education, so that parochial schools would not reap any benefit.

Nonetheless, in the 1970s, social issues drove many Catholics to question their steadfast loyalty to the Democratic Party. By the 1980s, many were voting Republican, due not only to social issues but also to their own improved economic and social status. Evangelical-led organizations, such as the Christian Coalition, in the 1990s made Catholic outreach a priority in their efforts at building support.

Leaders of both faiths cemented their surprising partnership on paper in the 1995 document “Evangelicals and Catholics Together,” signed by major figures in both communities who pledged to work together on such issues as abortion and government aid to religious schools.

In the 2000s, President George W. Bush made outreach to conservative Catholics a centerpiece of his strategy to build a broad base of support among the faithful. He weaved familiar Catholic discourse into many of his speeches to showcase his support for much of what traditional Catholics believe. In 2004, the famous “values voters” election, evangelicals and Catholics alone delivered 36 million of Bush’s 60 million votes.

Considering that Catholics were for decades solid Democratic Party voters, and that Catholics and evangelicals once stood in opposition in both theology and politics, this transformation was remarkable. Even in the Barack Obama elections, the Republican nominees held their strong support among evangelicals and achieved near parity with Catholic voters. The alliance very marginally weakened, but that’s all.

This year, the decline in Republican support among Catholics is dramatic. What is going on here?

Throughout the long years of partnership, Catholics have continued to hold less conservative positions than white evangelicals on many issues: immigration, the death penalty, health care, social welfare and more.

Trump has pushed a lot of these Catholics over the edge, especially with his strident immigration stands. Issues surrounding Trump’s personal conduct also have turned away some leading conservative Catholics. The conservative group CatholicVote lambasted Trump’s “character and moral judgment.” Dozens of conservative Catholic thought leaders signed a letter that declared Trump “manifestly unfit to be President of the United States” and said that he does not represent Catholic values.

Consider the Catholic beliefs about respecting the inherent dignity of every person. Trump’s controversial statements about numerous groups, and his mocking of a man with a disability, stand in sharp contrast to the culture that Catholics strive for. Indeed, many Catholics are disturbed to find that despite all of this, many evangelical leaders and voters can easily accept Trump.

To be sure, some prominent evangelical leaders, such as Russell Moore, the president of the Southern Baptist Convention’s Ethics & Religious Liberty Commission, and former Moral Majority leader Michael Farris have come out strongly against Trump. Farris even declared a meeting of Trump and evangelical leaders “the end of the Christian Right.”

But white evangelical voters are standing firm for Trump. That is a significant contrast with the many normally Republican-voting Catholics opposing Trump.

For four decades, political and religious leaders worked hard to bring together evangelical and Catholic voters on key moral issues. They achieved for some time a truly powerful alliance. The Trump candidacy now threatens to pull these groups back away from each other.

Mark J. Rozell is dean of the Schar School of Policy and Government at George Mason University and author of numerous studies on the intersection of religion and politics.